By Josh Perry, Editor
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Researchers at the University of California, Berkeley became the first to take an image of the microscopic state of negative capacitance in action and this insight could enhance energy-efficient electronics, according to a report from the university.

This image shows negative capacitance in action.
(Pablo Garcia Fernandez and Javier Junquera, Universidad de Cantabria)

“Capacitors are simple devices that can store an electrical charge,” the report explained. “Their capacitance, or ability to store electrical energy, is determined by how much the capacitor’s charge changes when it is connected to a voltage source, like a battery. Negative capacitance occurs when a change in charge causes the net voltage across a material to change in the opposite direction; so that a decrease in voltage leads to an increase in charge.”

Scientists believe that taking advantage of the relationship between charge and voltage could enhance voltage across dielectric materials to reduce the amount of voltage that a transistor would need, which would in turn make computers more energy-efficient.

The researchers captured negative capacitance in “an atomically perfect” lattice of ferroelectric-dielectric materials that was designed at the university.

“Using state-of-the-art imaging techniques, the researchers mapped out the polarization as well as the electric field with atomic resolution,” the article continued. “This allowed them to estimate the local energy density, which clearly showed regions where the curvature of the energy density is negative, indicating stabilization of the steady-state negative capacitance.”

The research was recently published in Nature. The abstract stated:

“Negative capacitance is a newly discovered state of ferroelectric materials that holds promise for electronics applications by exploiting a region of thermodynamic space that is normally not accessible. Although existing reports of negative capacitance substantiate the importance of this phenomenon, they have focused on its macroscale manifestation.

“These manifestations demonstrate possible uses of steady-state negative capacitance—for example, enhancing the capacitance of a ferroelectric–dielectric heterostructure or improving the subthreshold swing of a transistor. Yet they constitute only indirect measurements of the local state of negative capacitance in which the ferroelectric resides.

“Spatial mapping of this phenomenon would help its understanding at a microscopic scale and also help to achieve optimal design of devices with potential technological applications.

“Here we demonstrate a direct measurement of steady-state negative capacitance in a ferroelectric–dielectric heterostructure. We use electron microscopy complemented by phase-field and first-principles-based (second-principles) simulations in SrTiO₃/PbTiO₃ superlattices to directly determine, with atomic resolution, the local regions in the ferroelectric material where a state of negative capacitance is stabilized.

“Simultaneous vector mapping of atomic displacements (related to a complex pattern in the polarization field), in conjunction with reconstruction of the local electric field, identify the negative capacitance regions as those with higher energy density and larger polarizability: the domain walls where the polarization is suppressed.”